Rework left sidebar

* Move Buttons from left sidebar to JSTree
* Use light style for collapse sidebar buttons

lib/file_tree.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class FileTree < Tree::TreeNode
+class FileTree
   def file_icon(file)
     if file.file_type.audio?
       'fa fa-file-audio-o'
@@ -26,9 +26,11 @@ class FileTree < Tree::TreeNode
   private :folder_icon
 
   def initialize(files = [])
-    super(root_label)
+    # Our tree needs a root node, but we won't display it.
+    @root = Tree::TreeNode.new('ROOT')
+
     files.uniq(&:name_with_extension).each do |file|
-      parent = self
+      parent = @root
       (file.path || '').split('/').each do |segment|
         node = parent.children.detect {|child| child.name == segment } || parent.add(Tree::TreeNode.new(segment))
         parent = node
@@ -60,15 +62,10 @@ class FileTree < Tree::TreeNode
   end
   private :node_icon
 
-  def root_label
-    I18n.t('exercises.editor_file_tree.file_root')
-  end
-  private :root_label
-
   def to_js_tree
     {
       core: {
-        data: map_to_js_tree(self),
+        data: @root.children.map {|child| map_to_js_tree(child) },
       },
     }.to_json
   end